That's going to depend on a lot of factors:
1) What's your budget?
2) What are you going to be displaying the image on (wall, bed sheet, screen...)?
3) What are the room dimensions?
4) How much Throw Distance (the distance from the lens of the projector to the screen) do you have or want?
5) Are you able to control the lighting in the room you're going to use the projector in?
And that's just getting started.
-Wolf sends